1. Field of the Invention
The present invention relates generally to a method and apparatus for providing power to remotely located units using power supply lines or cables connected directly to central equipment serving the remote units.
2. Description of the Related Art
In many telecommunication systems, it is advantageous to power remotely located units, such as remote radio units, using power supply lines or cables connected to central equipment serving the remote units. The power from the central unit is typically converted by a power supply in the remote unit to provide the power necessary for the remote unit. This arrangement is generally more effective and less costly than using an external power supply for the remote units. In using central equipment to supply power to remotely located units, however, specific requirements must be met. Namely, the current and voltage supplied to the remote units must be strictly controlled. In addition, the power and heat dissipation of the conversion power supply in the remote units are limited and the electrical noise generated in the remote units must be kept extremely low. Further, the cost of the power supply in the remote unit for converting the power supplied from the central equipment must be kept low.
A key factor limiting the effective control of the aforementioned requirements is the variable line length of the cables connecting the central equipment to the remote units. The distance from the power source in the central equipment to each remote unit connected to the central equipment may vary. As such, the length of the cables necessary for connection of the central unit to each remote unit will vary accordingly. Because of the variable length of cables that may be required for each remote unit, the impedance of the power delivery system differs from remote unit to remote unit. This limits the effective control of the current and voltage applied to each remote unit.
With reference to FIG. 1, a typical power delivery arrangement exhibiting the aforementioned limitations is shown. An in-building wireless telecommunication system is an exemplary system in which power delivery systems may be used. A power source 14 is controlled by central unit 12 to provide power to power supply 22 of remote unit 20. In a wireless telecommunication system, remote unit 20 may be a remote radio unit for transmission of signals via antenna 23. In this arrangement, power is transmitted from power source 14 to power supply 22 via power transmission lines 16 and 18. Currently, power conversion in remote unit 20 is accomplished using either a linear power supply or a switching power supply within remote unit 20. Each of these power supplies has distinct disadvantages which limit the effective control of power supplied to remote units. With reference to FIG. 2, a linear power supply 28 is illustrated in which power is converted as it passes from the input 24 to the supply 26. The use of a linear power supply 28 at the remote unit 20 may have a very inefficient power conversion, and may also be quite dependent on the voltage drop across the supply cables. This tends to limit the line length of the cables. In addition, there is often quite a bit of heat dissipation associated with such an approach.
With reference to FIG. 3, a switching power supply 34 is shown in which power is converted as it passes from input 30 to supply 32. A switching power supply 34 is advantageous over a linear power supply 28 because it is more tolerant of large swings in input voltage. However, switching power supplies tend to be more expensive, and they produce considerable electrical noise.
Thus, there exists a need for an inexpensive, efficient power delivery system that provides power to remotely located units from a centrally located power source which includes a calibration system that can effectively compensate for voltage drops caused by the impedance of the supply cables.